1. Technical Field
The present invention relates to a microelectromechanical gyroscope with continuous self-test function, and to a method for controlling a microelectromechanical gyroscope.
2. Description of the Related Art
As is known, the use of microelectromechanical systems (MEMS) has increasingly spread in various technological sectors and has yielded encouraging results especially in providing inertial sensors, micro-integrated gyroscopes, and electromechanical oscillators for a wide range of applications.
MEMS systems of this type are usually based upon microelectromechanical structures comprising at least one mass, connected to a fixed body (stator) through springs and movable with respect to the stator according to pre-set degrees of freedom. The movable mass and the stator are capacitively coupled through a plurality of respective comb-fingered and mutually facing electrodes so as to form capacitors. The movement of the movable mass with respect to the stator, for example on account of an external stress, modifies the capacitance of the capacitors, whence it is possible to trace back to the relative displacement of the movable mass with respect to the fixed body and consequently to the force applied. Instead, by supplying appropriate biasing voltages, it is possible to apply an electrostatic force to the movable mass for setting it in motion. In addition, for providing electromechanical oscillators, the frequency response of MEMS inertial structures is exploited, which is typically of a second-order low-pass type, with one resonant frequency.
In particular, MEMS gyroscopes have a more complex electromechanical structure, which comprises two masses that are movable with respect to the stator and are coupled to one another so as to have a relative degree of freedom. The two movable masses are both capacitively coupled to the stator. One of the masses is dedicated to driving and is kept in oscillation at the resonant frequency. The other mass is driven in (translational or rotational) oscillatory motion and, in the event of rotation of the microstructure with respect to a pre-determined gyroscopic axis with an angular velocity, is subject to a Coriolis force proportional to the angular velocity itself. In practice, the driven mass, which is capacitively coupled to the fixed body through electrodes, as likewise the driving mass, operates as an accelerometer that enables detection of the Coriolis force and acceleration and hence of the angular velocity.
As practically any other device, MEMS gyroscopes are subject to production defects (which can regard the entire microstructure and the electronics) and wear, which can reduce the reliability thereof or jeopardize their operation completely.
For this reason, prior to being installed, gyroscopes undergo testing in the factory for proper operation, which enables identification and rejection of faulty examples.
In many cases, however, it is important to be able to carry out sampled checks at any stage of the life of the gyroscope, after its installation. In addition to the fact that, in general, it is advantageous be able to locate components affected by faults, to proceed to their replacement, MEMS gyroscopes are used also in critical applications, where any malfunctioning may have disastrous consequences. Just to provide an example, in the automotive sector the activation of many air-bag systems is based upon the response provided by gyroscopes. It is thus evident how important it is to be able to equip MEMS gyroscopes with devices that are able to carry out frequent tests on proper operation.
It should moreover be noted that the circuits for testing do not have to affect significantly the overall dimensions and the consumption levels, which assume increasing importance in a large number of applications.
In this connection, solutions have been developed that enable execution of in-field tests for proper operation in given conditions. According to known solutions, in particular, a self-test signal is generated starting from driving signals used for keeping the driving mass in oscillatory motion. The self-test signal is synchronous with the oscillations of the driving mass and is supplied to electrodes for self-testing of the sensing mass, which are configured so as to apply electrostatic forces to the sensing mass in the sensing direction, in response to the self-test signal. In practice, then, the self-test signal causes an effect that is altogether similar to that of a rotation of the microstructure about the gyroscopic axis. The amplitude of the self-test signal is known. In conditions of absence of any rotation of the microstructure, it is hence possible to detect the response of the gyroscope and, by comparing it with an expected response, determine whether the gyroscope functions properly or whether any malfunctioning has arisen.